In order to produce a laser beam, light is introduced into a host material which contains an active ingredient, to raise the atoms of the active ingredient to higher energy levels. When a number of these active atoms return to their original energy state, they do so in phase and produce radiation of the same frequency, thus producing a laser beam. The first laser action was produced using chromium as the active ingredient in a ruby host material. Later, lasing action was produced by neodymium contained in a glass host material. One system of current importance in laser technology uses an active ingredient of neodymium (Nd) and a host material of yttrium aluminum garnet (YAG), as discussed by A. E. Siegman in the book entitled "An Introduction to Lasers and Masers," McGraw-Hill Book Company, New York, 1971 and also in U.S. Pat. No. 4,013,501. Neodymium:YAG lasers are particularly useful for the application of optical methods to measure range, such as in range finders and in target designators and target illuminators.
One of the objectives of laser technology has been to increase the lasing efficiency or fluorescence yield of a laser so that the laser energy output can be maximized for a given input energy. Initial measurements by S. Singh, R. G. Smith and L. G. Van Uitert, in a publication entitled "Stimulated-emission cross section and fluorescent quantum efficiency of Nd.sup.3+ in yttrium aluminum garnet at room temperature," Physical Review B, Vol. 10, No. 6, 15 Sept. 1974, pp. 2566-2572, indicated that the absolute radiative quantum efficiency of the .sup.4 F.sub.3/2 state of Nd.sup.3+ in YAG was 56 percent. Further work by S. Singh, W. A. Bonner, W. H. Grodkiewicz, M. Grasso, and L. G. Van Uitert, reported in a publication entitled "Nd-doped yttrium aluminum garnet with improved fluorescent lifetime of the .sup.4 F.sub.3/2 state", Applied Physics Letters, Vol. 29, No. 6, 15 September 1976, pp. 343-345, aimed at improving the fluorescent lifetime of the metastable level of the laser transition, approached this problem by preparing Nd-doped YAG polycrystals in powdered form. Another approach to improving lasing efficiency was taken by J. W. Strozyk as described in a publication entitled "Lutetium effects on the UV absorption spectra of Nd:YAG", in IEEE Journal of Quantum Electronics (Correspondence), Vol. QE-7, Sept. 1971, pp. 467-469, where lutetium was used to promote the activity of the Nd atoms and thus improve lasing efficiency. Such an addition of certain metal ions to a laser material to improve lasing properties has been known in the art. For example, chromium ions have been added to a neodymium-doped laser material so that the chromium ions can absorb energy and transfer it to the neodymium ions, thereby permitting excitation of the neodymium ions over a wider energy band, as discussed by R. C. Ohlmann and R. Mazelsky, in the publication entitled "Energy transfer from Cr.sup.3+ to Nd.sup.3+ in solids" in Physics of Quantum Electronics, San Juan, Puerto Rico, McGraw-Hill, New York, 1966, pp. 322-331. In addition, it has been known that lasing efficiency can be improved by removing from the laser material metal ion impurities which quench the lasing action, as discussed by L. A. Riseberg and M. J. Weber, in "Relaxation phenomena in rare earth luminescence," in Progress in Optics, edited by E. Wolf, Elsevier North Ireland, New York, 1976.